1. Field of the Invention
The present invention relates to an optical fiber coupling part for coupling a light emitting source such as a semiconductor laser used for optical communication and an optical fiber with high coupling efficiency.
2. Description of the Related Art
A technique for coupling a semiconductor laser and an optical fiber with high coupling efficiency is one of the most important techniques in optical communication. For example, conventional methods of coupling the semiconductor laser and the optical fiber include a method using a lens such as a spherical lens and an aspherical lens, or a method using a tip ball fiber whose tip part is spherical (see U.S. Pat. No. 3,910,677). The method using the lens involves problems in that mutual alignment of the optical axes of the semiconductor laser, the lens, and the optical fiber is complicated, enlarging the entire body of the coupling system at the same time, thereby increasing the manufacturing cost, while relatively high coupling efficiency is obtained. In addition, since the dimension of the lens is large and a disposition space is thereby largely occupied, this method can not be used for coupling a semiconductor laser array and an optical fiber array in which a plurality of semiconductor lasers and a plurality of optical fibers are arranged at short intervals. Meanwhile, since formed in small size, the method using the tip ball fiber is capable of coupling the semiconductor array and the optical fiber array. The aforementioned optical fiber is integrally formed with a hemispherical lens part at the tip of a single mode optical fiber. Meanwhile, when making the tip ball optical fiber, a problem is that conventionally the tip part of the fiber is polished around, and therefore mass-productivity is deteriorated and it takes significant labor hours to produce. Another problem is that, since the tip of the optical fiber is spherical, coupling efficiency is deteriorated due to spherical aberrations. Specifically, light beam emitted from a laser end face reaches the end face of the single mode optical fiber at different positions and at different angles, depending on the exiting angle of the outgoing light. Therefore, some of the light beams deviate from the core, or even when it reaches the core, an incident angle to the core is equal to or larger than a critical angle, and therefore the light is not propagated through the single optical fiber to deteriorate coupling efficiency. For example, when a standard semiconductor laser is used, the coupling loss is approximately 6 dB.
In order to solve the above-described problems, a columnar graded index lens (called “GRIN lens” hereafter) facilitating optical axis alignment is used. The GRIN lens is the lens using medium materials not uniform in refractive index (refractive index becomes larger toward the center of the lens), and functions as a lens whose refractive index is continuously changing. Refractive index distribution n (r) of the GRIN lens in a radius direction is expressed by the following formula:n(r)=n0(1−(½)(gr)2)(FIG. 1)where, n (r) is a constant expressing the refractive index at distance r from the center, n0 is a constant expressing the refractive index at the center part, and g is a constant expressing a light-condensing performance of the GRIN lens. The above-described lens has relatively small spherical aberrations. However, the critical angle of the conventionally existing GRIN lens is made small, set to be 20° or less. Thus, such a lens can not sufficiently take in lights emitted from the semiconductor laser for optical communication, whose standard radiation full angle at half maximum θ is approximately 25°, thereby increasing the coupling loss. Therefore, a ball lens and the GRIN lens are frequently used in combination. However, in this case, it is difficult to align the optical axis, thereby increasing the assembly cost. Moreover, it is designed so that the tip of the GRIN lens is cut in spherical shape to increase NA in appearance (to improve the light-condensing performance). However, the problems are that mass-productivity is deteriorated and it takes significant labor to produce, thereby increasing the manufacturing cost. Further, the GRIN lens is conventionally made of a multi-component glass, and its softening point is about 500 to 600° C. Therefore, such a GRIN lens can not be fusion-spliced with the optical fiber, which is mainly composed of quartz glass. Thus, an optical adhesive is used, thereby posing problems in that it is difficult to align the optical axis, and an optical characteristic is deteriorated by a change in the quality of the adhesive caused by temperature-raise, when the adhesive absorbs the light and high intensity light thereby enters.
In order to solve such a problem of connection deterioration, a structure using GI (Graded-Index) optical fiber as a lens has been proposed (see U.S. Pat. Nos. 4,701,011 and 5,384,874).
The GI optical fiber is the optical fiber in which the refractive index of a core part changes in a radial direction. Since the GI optical fiber is made of the same quartz as the optical fiber, the GI optical fiber can be fusion spliced with the optical fiber. Therefore, it can be expected that the GI optical fiber will have high durability against light of high intensity. However, in this case, the critical angle of the GI optical fiber is made small, set to be 20° C. or less, (light-condensing performance is small), and therefore it is difficult to sufficiently take in the light emitted from the semiconductor lens for optical communication, whose standard radiation full angle at half maximum 0 is approximately 25°, and therefore the coupling loss is large, and handling property is low when actually assembled as a lens.
In order to solve the above-described problems, it is desired to develop a GRIN lens having light-condensing performance (high numerical aperture) which is high enough to sufficiently cover the emission angle of a semiconductor laser. Particularly, a standard radiation full angle at half maximum of a semiconductor laser is equal to 25° or larger. Therefore, in order to sufficiently guide the light of the semiconductor laser to the GRIN lens, it is necessary to develop a GRIN lens having a critical angle of at least 25° or larger. The critical angle corresponds to a maximum angle formed with an axis which allows the light to enter the optical fiber and the GRIN lens, when the light enters the optical fiber and the GRIN lens at an angle relative to the axis. Usually, a sine function of the critical angle is referred to as numerical aperture (referred to as “NA” hereafter). When the radiation full angle at half maximum of the semiconductor laser is 25°, the numerical aperture NAs is 0.43. Therefore, when the GRIN lens having NA which is equal to 0.43 or larger is used, all the lights of the semiconductor laser can enter the lens. Thus, such a GRIN lens is required. In addition, in order to facilitate the optical alignment of the axes of the semiconductor laser, the GRIN lens, and the optical fiber, a coefficient of thermal expansion of the GRIN lens needs to be set at 15×10−7K−1 or less, while the coefficient of thermal expansion of quartz is set at 5×10−7K−1. The above-described fusion splicing is a required technique for improving productivity, and by the fusion-splicing, the light reflected from a boundary surface between the optical fiber and the lens and returned to the semiconductor laser is reduced, to solve the problem that the optical characteristic is deteriorated by a change in the quality of the adhesive caused by temperature-raise, when the adhesive absorbs the light, and high intensity light enters. In addition, if the optical fiber and the GRIN lens having approximately the same sectional shape are fusion-spliced under flame using an oxyhydrogen burner, etc., due to a self-aligning effect (effect that center axes of both of the optical fiber and the GRIN lens are naturally coincident with each other by the surface tension of fused glass), the center axes of the optical fiber and the lens are coincident with each other without accurate axis aligning which has been a long-pending problem, thereby obtaining a large advantage in that the assembling property is significantly improved.
A method of efficiently condensing the light of the semiconductor laser by using the GRIN lens having high light-condensing performance as described above, includes the method of directly fusion-splicing the GRIN lens having high NA with the tip of the optical fiber. However, in this case, the coupling loss of about 3-4 dB must be expected. The reason is that, although the light radiated from the end face of the semiconductor laser is condensed on the end face of the single mode optical fiber by the light-condensing effect of the GRIN lens having high NA, a part of the light having a large emission angle reaches an angle that is larger than the critical angle of the optical fiber. Particularly, the problem is that, when the critical angle of the semiconductor laser (sine function of this critical angle=numerical aperture called NAs) is larger than the critical angle of the optical fiber (sine function of this critical angle=numerical aperture called NAf), the light deviates from the core of the optical fiber depending on the emission angle of the light beam, or even if the light reaches the core, the incident angle to the core is equal to the critical angle or larger, thereby failing with regard to entering the single mode optical fiber and deteriorating the coupling efficiency.
In order to solve the above-described problem, an optical fiber with lens has been proposed (see Japanese Patent Laid Open No. 8-292341). In the optical fiber with lens, one end of the single mode optical fiber having a core and a clad and the other end of the coreless optical fiber are connected by a 2nd square type optical fiber (corresponding to GRIN lens). The 2nd square type optical fiber has a 2nd square type refractive index distribution of length of nearly ¼ as long as a zigzag cycle of a light beam propagated through the lens or the length of an odd number times of the length of ¼ of the zig-zag cycle. The optical fiber with lens is formed by connecting the 2nd square type optical fiber (corresponding to GRIN lens) having 2nd square type refractive index distribution of length of nearly ¼ as long as a zigzag cycle of a light beam propagated through the lens or the length of an odd number times of the length of the ¼ of the zig-zag cycle, to the single mode optical fiber having the core and the clad. Here, the 2nd square type optical fiber has the core and the clad, and the tip is formed in semi-spherical shape. By using the above-described optical fiber, the coupling loss is reduced to approximately 4 dB when coupled to the semiconductor laser, which is not enough to satisfy the coupling loss (3 dB or less) required practically. Generally, the smaller the coupling loss of the semiconductor laser and the optical fiber, the higher the performance of an optical communication system becomes, thereby also facilitating system construction. In addition, the tip is formed in semi-spherical shape with low yield ratio at a high cost. In order to not significantly lower the coupling efficiency of the lens having semi-spherical tip and the semiconductor laser, distance between the semi-spherical lens and the semiconductor laser, in other words, operating distance must be approximately 10 μm. Therefore, a disadvantage is that when constructing a coupling system of coupling the optical fiber with semi-spherical lens and the semiconductor laser, the semiconductor laser and the semi-spherical lens collide with each other, resulting in being unusable.
However, a conventional technique relating to the optical fiber with lens can not simultaneously satisfy requirements such as realizing more reduced coupling loss, further facilitating aligning of the optical axes of a semiconductor laser, a lens, and an optical fiber, while maintaining a long operating distance. In view of the above-described problems, the present invention is provided, and an object of the present invention is to provide the optical fiber with a GRIN lens and a laser module capable of reducing the coupling loss while maintaining a long operating distance and having a good module assembling property.